Both integrated circuit (IC) device fabrication and microelectromechanical systems (MEMS) fabrication make use of one or more layers of particular coatings deposited on, and typically chemically bonded to a surface of, a substrate for various purposes.
In some instances, such as when the one or more coating layers are used as a patterned masking material (e.g. by photolithography), the one or more layers are deposited on a substrate and subsequently removed after the pattern is transferred to an underlying layer.
In other instances, the one or more coating layers are deposited to perform a function in a device or system and remain as part of the fabricated device, e.g. by etching.
The small (often nanometer) size scale of some IC devices and MEMS means they are suitable for use in a wide range of applications including inkjet printers, accelerometers, gyroscopes, pressure sensors, displays, optical switching technology, biological sciences, and the like. Such uses means that the type and properties of the one or more coating layers on the substrate surface are chosen to provide a particular functionality to the surface, typically by provision of specific functional moieties.
It is known to be desirable to provide one or more uniform, thin coating layers, such as silane coating layers, on substrate surfaces, especially silicon substrate surfaces. Those working in the IC device and MEMS fields have recognized the advantages of vapour-deposited coatings (i.e. coatings formed when a chemically reactive species present in a vapour is reacted with a surface of a substrate) over coatings applied using liquid-based immersion, spray-on and spin-on techniques, including: elimination of stiction (static friction) induced by capillary forces; control of the coating environment (particularly the amount of moisture present); provision of an extremely smooth coating surface without any detectable number of sub-micron aggregates; consistent uniform coating properties with micron- and nanometer-sized patterns such as microchannels and pores; solvent-free processing with no contamination; and faster processing which is compatible with MEMS clean room processing protocols.
Vapour deposition of coatings is particularly useful for deposition of thin coatings having a thickness ranging from around 5 Å to around 1000 Å (although may be used for increased coating thicknesses up to around 2000 Å). In particular, vapour deposition can be used for the preparation of a self-assembled monolayer (SAM) of a particular chemical species, i.e. an organised layer of amphiphilic molecules in which one end of the molecule (the “head group”) becomes chemisorbed onto the surface of a substrate, whilst the other end of the molecule (the “tail group”), which may be provided with a terminal functional group, achieves two-dimensional organisation until the substrate surface is covered in a single, orderly, monolayer of molecules.
In the context of IC devices and MEMS it is known to provide a silicon substrate with a silane SAM, with the head group of the silane being hydrophilic and strongly chemisorbed to a surface of the silicon substrate, and the tail group (connected to the head group by an alkyl chain) being hydrophobic to provide desired wetting and interfacial properties. For example, with certain MEMS, a hydrophobic surface (coating) is needed to prevent adhesion of adjacent MEMS surfaces due to capillary forces in water.
Despite the many advances that have been made in the field of fabrication of IC devices and MEMS, there remains a need to protect, or further protect, the surface(s) of these entities from environmental influences, e.g. humidity, to ensure their reliable, long-term performance. There is also a desire to be able to functionalise, or further functionalise, the surface(s) of these entities in an application-specific manner, e.g. to provide a surface which is sensitive to protein absorption.